Variable impedance network with coarse and fine controls

ABSTRACT

An impedance network. The network includes a plurality of impedance elements, at least one end terminal, and a wiper terminal. The network also includes a first plurality of switching elements selectively providing tap positions to the at least one end terminal, selectable at a first specified increment of impedance elements in the plurality of impedance elements. The network further includes a second plurality of switching elements selectively providing a tap positions to the wiper terminal, selectable at a second specified increment of impedance elements in the plurality of impedance elements.

BACKGROUND

The present invention relates to a variable impedance network. Moreparticularly, the invention relates to such a variable impedance networkwith coarse and fine controls.

Variable impedance networks are usually manually adjusted to provide aselected impedance so as to affect some aspect of the circuit in whichthe networks are located. These variable impedance networks are usuallyin the form of variable resistors, also called potentiometers. However,circuits using variable inductors or capacitors may also be formed.

Manual adjustment of potentiometers is usually undesirable in circuitsunder the control of data processing systems or other external electriccircuits where ongoing adjustment of the potentiometer is necessary forcircuit operation. The data processing system often must change thevalue of the variable impedance network in a time that is short relativeto the time required to complete a manual adjustment of the variableimpedance element. Therefore, special purpose integrated circuitvariable impedance networks have been employed in the prior art. Thesenetworks allow the level of attenuation to be adjusted under the digitalcontrol of an external data processing system.

For example, Tanaka, et al., U.S. Pat. No. 4,468,607, teaches a ladderattenuator which is controlled by a binary number by means of a switchcircuit. Depending on the stage of the switches in this switch circuit,one or more stages of attenuation are introduced into the signal path.However, teachings of Tanaka may require a large number of fixedimpedance elements and switches for a large range of impedances.Accordingly, Drori, et al., U.S. Pat. No. 5,084,667, suggests a numberof embodiments of variable impedance elements which minimizes the numberof separate resistors required to achieve the equivalent resolutionachievable using a series arrangement of resistors.

SUMMARY

The present invention, in one aspect, describes an impedance network.The network includes a plurality of impedance elements, at least one endterminal, and a wiper terminal. The network also includes a firstplurality of switching elements selectively providing tap positions tothe at least one end terminal, selectable at a first specified incrementof impedance elements in the plurality of impedance elements. Thenetwork further includes a second plurality of switching elementsselectively providing a tap positions to the wiper terminal, selectableat a second specified increment of impedance elements in the pluralityof impedance elements.

In another aspect, the present invention describes a method forconfiguring an impedance network. The method includes providing aplurality of impedance elements, providing at least one end terminal anda wiper terminal, first selectively providing tap positions to the atleast one end terminal, selectable at a first specified increment ofimpedance elements in the network, and second selectively providing atap positions to the wiper terminal, selectable at a second specifiedincrement of impedance elements in the network.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A shows a conventional variable resistance network.

FIG. 1B shows a standard center-tapped potentiometer.

FIG. 2A shows another conventional variable resistance network.

FIG. 2B illustrates a variable impedance network in accordance with anembodiment of the invention.

FIG. 2C illustrates a variable impedance network in accordance with analternative embodiment of the invention.

FIG. 3 illustrates a new method for configuring an impedance networkarray in accordance with an embodiment of the present invention.

FIG. 4 depicts one embodiment of the network schematic of the newconcept, dubbed “A-W-B” as implemented for a potentiometer with 256equally discernible steps.

FIG. 5 shows a range change system, dubbed “A-W-B” as implemented for apotentiometer with 256 equally discernible steps, according to oneembodiment of the invention.

FIG. 6 illustrates a “range move down” simulation according to anembodiment of the invention.

DETAILED DESCRIPTION

In recognition of the above-stated challenges associated with prior artdesigns of variable impedance networks, alternative embodiments for avariable impedance network, which reduces overhead circuits and enhancesoperation with coarse and fine controls, are described. The network andits associated control method use coarse and fine wiper control forbuilding potentiometers and digital-to-analog converters (DACs). In thisscheme, the wiper terminal and the two end terminals are allowed to beprogrammably movable. The wiper terminal is allowed to connect in finesteps in a section of the network. The two end terminals are switchedtogether in coarse steps while maintaining a constant total resistance.Smooth transition between coarse step is made possible by turning thecorresponding switches to end-terminals on and off in stages, a portionat a time. A significant reduction of chip area may be achieved withlittle or no degradation of chip performance. Consequently, for purposesof illustration and not for purposes of limitation, the exemplaryembodiments of the invention are described in a manner consistent withsuch use, though clearly the invention is not so limited.

A conventional variable resistance network 100 is illustrated in FIG.1A. The network 100 includes a counter 102, a control circuitry 104, adecoder 106, and a network array 108 having a transistor array 110 and aresistor array 112. In the illustrated example, the network array 108has three terminals, H, L and W. Hence, the network array 108 simulatesa standard potentiometer 120, such as the one shown in FIG. 1B.Terminals H and L correspond to the end terminals, while terminal Wcorresponds to the center tap of the potentiometer 120.

In the illustrated example of FIG. 1A, the resistor array 112 includes32 equal resistor elements (R) arranged in series to represent 32 tappositions at the wiper nodes of the potentiometer 120. However, anynumber of resistor elements may be used to provide smaller or largerresistance value than this example. The transistor array 110 includeswiper transistors that are used to connect various combinations ofresistor elements between two terminals H and W.

The particular combination is determined by a value stored in a counter102, which may be altered by two signals, U/D and INCR. The U/D signaldetermines whether the counter 102 will be incremented or decremented bya predetermined amount in response to the increment (INCR) signal. Thisvalue is coupled to a 1-of-N decoder 106, where N=32. The output of thisdecoder 106 controls the plurality of wiper transistors in thetransistor array 110. Since N is the maximum value which may be storedin the counter 102, there are N nodes in the resistor array 112, eachnode corresponding to a given counter value. Each node may be coupled toterminal W by applying a signal to the corresponding wiper transistor inthe transistor array 110.

The value stored in the counter 102 may be transferred to a memory inthe control circuitry 104 in response to specified voltage transitionson a chip select (CS) line. The chip select line also enables thecounter 102. When the chip select line is low, the counter 102 respondsto signals on U/D and INCR lines. This enables the circuit controllingthe variable resistance network 100 to alter the value stored in counter102.

The control circuitry 104 also monitors supply voltages (V_(cc) andV_(ss)) to load the value stored in the memory into the counter 102 whenpower is applied to the variable resistance network 100. This ensuresthat the last value stored in counter 102 before power was removed fromthe variable resistance network 100 will be restored when the power isonce again applied to the variable resistance network 100.

With the above-described approach illustrated in FIGS. 1A and 1B, Nwiper transistors are required to generate N tap positions. Hence, whenN becomes large (e.g., N>100), the area of the die occupied by the wipertransistors may significantly increase, especially when thespecification for wiper resistance is low (i.e., 50 ohms or less).

Accordingly, the present embodiments include solutions to theabove-stated undesirable outcome of large N by providing a variableimpedance network which requires fewer wiper transistors. Moreover, theteachings of these embodiments may be extended to include impedancenetworks having elements other than resistors, such as capacitors orinductors. In the below-described embodiments, the impedance network isa binary numbering scheme assigned to a plurality of serially connectedresistive pairs, where each pair is connected in parallel. However, inan alternative embodiment, more than two resistors may be configured inparallel arrangement to provide wider range of resistance values, andthus, further reduce the wiper transistor count. In a furtherembodiment, bypass transistors may be provided to bypass certainresistors. This may also provide wider range resistance values.

In a conventional network shown in FIG. 2A, wiper contacts, labeled as“W”, are brought out at every step, where each step represents theresistor element for the finest increment of resistance value. Thisconfiguration is substantially similar to the earlier conventionalconfiguration 100 mentioned above. The end contacts, labeled as “H” and“L”, are fixed in this configuration.

FIG. 2B illustrates a variable impedance network 200 in accordance withan embodiment of the invention. Hence, in FIG. 2B, the wiper contacts,labeled as “W”, are brought out as “fine adjustment”, for only onesection 202 of the resistor string. To accommodate moving end contacts,the base string are lengthened to approximately twice the lengthrequired for fixed end contacts (i.e., two times the required length forfixed end contacts minus the length of the section of “wipers”). Then“H” end contacts are introduced (with a regularity of the length of thesection of “wipers”), to the upper side 204 of the “wiper” section 202.And “L” end contacts are introduced (with regularity of the length ofthe section of “wipers”), to the lower side 206 of the “wiper” section202. Pairs of “H” and “L” contacts may be selected, such that theresistances remain constant, and the “wiper” section 202 appears at thedesired position. In this way, the “H”-“L” pair serves for range changeand the “wiper” section 202 serves for fine adjustment.

For example, to program zero to 4R between H and wiper terminals, coarsetap switches A4 and B4 are activated, and fine tap switches, W0 to W4,are successively activated. To program 5R to 8R between H and wiperterminals, coarse tap switches A3 and B3 are activated, and fine tapswitches, W0 to W4, are successively activated. To program 9R to 12Rbetween H and wiper terminals, coarse tap switches A2 and B2 areactivated, and fine tap switches, W0 to W4, are successively activated,and so on. Thus, it can be seen that resistance values at all incrementsteps may be programmed with a pair of coarse tap switches and a finetap switch.

FIG. 2C shows an alternative embodiment 210 of the coarse-fineresistance approach shown in FIG. 2B. In this embodiment, the coarsecontact points (pass gates) 212, 214 are not connected to the end point.A pair of fine tap resistor networks 216, 218 is substituted for the twocoarse resistance taps placed at the two ends of the network. Thissubstantially reduces the direct connection of pass gates to the endpoints of the resistor network 210. This approach provides additionaladvantages to the network 210. This embodiment may allow the entireresistor network 210 to be configured into three different adjustmentlevels where the middle resistor network selected by wiper pass gatesprovides intermediate adjustment, the two resistor network adjacent tothe middle resistor network provides coarse adjustment, and the resistornetworks connected to the two end terminals provides fine adjustment.

Advantages of this alternative embodiment 210 over the network 200 shownin FIG. 2B include the fact that the network 210 produces lesscapacitance seen at the end terminals since the pass devices are notdirectly connected to the end terminals. Thus, less capacitive couplingis introduced into the end terminals while changing the coarse switches.Also, the wiper resistance at the end terminal nodes passes through asingle pass device. Further, total network resistance characteristicssuch as Integral non-Linearity (INL) and differential non-linearity(DNL) may be designed to perform better with this type of network.

A new method for configuring an impedance network array in accordancewith an embodiment of the present invention is illustrated in FIG. 3.The method includes selectively connecting a first plurality ofresistors to the two end terminals of a variable impedance network, at300, for a coarse adjustment. At 302, a second plurality of resistiveelements is selectively connected to the wiper terminal for fineadjustment. Furthermore, the first and second pluralities of resistorsare configured to provide all increments of resistance value in thevariable impedance network, at 304.

Advantages of the new approach over the conventional approach describedabove include reduction the number of wiper transfer gates. Theconventional approach scales linearly with the number of taps. Hence,the conventional approach uses n+1 wiper/pass transistors for n neededtaps. The new approach described in conjunction with FIGS. 2B and 4Cscales with the number of taps as a function of square root. Therefore,this approach uses 3*{square root over (n)} pass/wiper transistors for nneeded taps. Other advantages include the ability to tap directly intothe resistor string without any additional resistance seen from thewiper terminal, other than that from the wiper pass gate. Furthermore,there are at most two additional pass gates between the two endterminals during operation. Another possible advantage is that theeffective parasitic capacitance induced by the pass gates is reduced,since the number of the pass gates is far less then the conventionalapproach. This increases the frequency response of the potentiometer.

The new approach presents some disadvantages including having to useapproximately 2 times more unit size resistors than the conventionalmethod. However, since the unit resistors are not the major areacontributor to the die size, the impact of this increase in the numberof unit size resistors may be overcome by the reduction of thepass/wiper transistor overhead, especially for potentiometers with largenumber of taps.

While specific embodiments of the invention have been illustrated anddescribed, such descriptions have been for purposes of illustration onlyand not by way of limitation. Accordingly, throughout this detaileddescription, for the purposes of explanation, numerous specific detailswere set forth in order to provide a thorough understanding of thepresent invention. It will be apparent, however, to one skilled in theart that the embodiments may be practiced without some of these specificdetails. In other instances, well-known structures and functions werenot described in elaborate detail in order to avoid obscuring thesubject matter of the present invention. Accordingly, the scope andspirit of the invention should be judged in terms of the claims whichfollow.

Appendix

The new resistive network that was constructed uses a fine and coarsescheme by shifting the end terminals together in coarse steps and bychanging the wiper terminal in fine steps, the conventional manner.

1. Network Architecture

FIG. 4 depicts one embodiment of the network schematic of the newconcept, dubbed “A-W-B” as implemented for a potentiometer with 256equally discernible steps. With R as the unit resistance, the totalresistance of the string is 256R and the wiper is supposed to tap intothe string in multiple of the unit, resistance. The resistor stringincludes three sections: the top (A) section—the section of moving “A”ends; the middle (W) section—the section of moving wipers; and thebottom (B) section—the section of moving “B” ends. Each section isdescribed in detail below.

In the left (A or the top) section, there are fifteen resistor segmentsof equal resistance (16R each) and sixteen tapping points, between theseresistor segments, labeled A0 to A15. From these tapping points,connections to the A-end terminal may be made. Specifically, the A0tapping point is hard-wired to the A-end terminal via resistor r₀. Forthe group of A1 to A15 tapping points, a switch is (not drawn) placedbetween each tapping point and the A-end, and with its resistancesymbolized in a pair of square brackets such as [r_(x)]. The resistancesof these switches are part of the overall path resistances are denotedsymbolically as r₁, . . . , r₁₅. Thus, A4 [r₄] in FIG. 4 reads “tappingpoint A4 of Section A with a switch of resistance r₄, connecting toA-end terminal”.

In the center (W or the middle) section, there are fourteen resistorsegments of equal resistance (R), and two end resistor segments withresistance of (R-r₀), and seventeen tapping points, between theseresistor segments, labeled W_(—)0 to W_(—)16. A switch (not drawn) isplaced between each tapping point and the wiper terminal. From thesetapping points, connections to the wiper W terminal may be made.

In the right (B or the bottom) section, there are fifteen resistorsegments of equal resistance (16R each) and sixteen tapping points,between these resistor segments, labeled B0 to B15. From these tappingpoints, connections to the B-end terminal may be made. Specifically, theB15 tapping point is hard-wired to the B-end terminal via resistor r₀.For the group of B0 to B14 tapping points, a switch (not drawn) isplaced between each tapping point and the B-end, and with its resistancesymbolized in a pair of square brackets such as [r_(x)]. The resistancesof these switches are part of the overall path resistances are denotedsymbolically as r₁₅, . . . , r₁.

This network is constructed to meet two major constraints. The firstconstraint is that the end-to-end resistance of the potentiometer shouldremain constant. The second constraint is that all possible taps need tobe generated at the wiper node (variable node of the potentiometer).

To accomplish the end-to-end constraints the A and B terminal sectionsare mirrored symmetrically with each other around the center wipersection so that the resistance between V_(A) and V_(W) is set to thedesired value of x times R by selecting the appropriate switches and theend-to-end resistance between V_(A) and V_(B) constant to 256R.

In addition to the resistors, r₁ to r₁₅, representing the switches, theresistors (R-r₀) and r₀ are introduced for compensating the effect ofnon-zero switch resistance. Depending on the operating scheme used, thevalues of r, r₀, r₁ to r₁₅ may be calculated so that the “correct”voltages appear at the wiper contacts. The circuit elements and theconnections required for the functioning of a potentiometer with 256steps are summarized in Table 1 as follows:

TABLE 1 The Circuit Elements and Connections for the Network TappingCircuit Connection Tapping Circuit Connecting Tapping Circuit Connectionpoint Element to point Element [a] to point. Element to A0 Resistor, r =r₀ A-end W_0 Resistor r₀ W-end B0 Switch, r-on = r₁₅ B-end A1 Switch,r-on = r₁ A-end W_1 Switch, r-on = r_(w) W-end B1 Switch, r-on = r₁₄B-end A2 Switch, r-on = r₂ A-end W_2 Switch, r-on = r_(w) W-end B2Switch, r-on = r₁₃ B-end A3 Switch, r-on = r₃ A-end W_3 Switch, r-on =r_(w) W-end B3 Switch, r-on = r₁₂ B-end A4 Switch, r-on = r₄ A-end W_4Switch, r-on = r_(w) W-end B4 Switch, r-on = r₁₁ B-end A5 Switch, r-on =r₅ A-end W_5 Switch, r-on = r_(w) W-end B5 Switch, r-on = r₁₀ B-end A6Switch, r-on = r₆ A-end W_6 Switch, r-on = r_(w) W-end B6 Switch, r-on =r₉ B-end A7 Switch, r-on = r₇ A-end W_7 Switch, r-on = r_(w) W-end B7Switch, r-on = r₈ B-end A8 Switch, r-on = r₈ A-end W_8 Switch, r-on =r_(w) W-end B8 Switch, r-on = r₇ B-end A9 Switch, r-on = r₉ A-end W_9Switch, r-on = r_(w) W-end B9 Switch, r-on = r₆ B-end A10 Switch, r-on =r₁₀ A-end W_10 Switch, r-on = r_(w) W-end B10 Switch, r-on = r₅ B-endA11 Switch, r-on = r₁₁ A-end W_11 Switch, r-on = r_(w) W-end B11 Switch,r-on = r₄ B-end A12 Switch, r-on = r₁₂ A-end W_12 Switch, r-on = r_(w)W-end B12 Switch, r-on = r₃ B-end A13 Switch, r-on = r₁₃ A-end W_13Switch, r-on = r_(w) W-end B13 Switch, r-on = r₂ B-end A14 Switch, r-on= r₁₄ A-end W_14 Switch, r-on = r_(w) W-end B14 Switch, r-on = r₁ B-endA15 Switch, r-on = r₁₅ A-end W_15 Switch, r-on = r_(w) W-end B15 Switch,r-on = r₀ B-end W_16 Switch, r-on = r_(w) W-end [a] r_(w) can be setequal to r_(o) or some other values depending on the system spec.

2. Operation Scheme

To set the wiper to a particular step, a pair of “A” and “B” tappingpoints is selected first, such that “wiper” section falls within theappropriate range. Then, the wiper contact is set to a specific “W”tapping point. Using the example of a potentiometer with 256 steps, asdepicted in FIG. 2B, the operation scheme may best be described in atable form. Table 2 shows how the A-B pairs should be selected as afunction of Wiper Step Number. The Wiper Step Number is the resistancein unit of R between terminal W and B-end.

TABLE 2 The Selection of A-End and B-End tapping points vs. Wiper StepNumbers Wiper A Section tapping points Step No. 0 1 2 3 4 5 6 7 8 9 1011 12 13 14 15 1  1-16 y  17-32 y y  33-48 y y  49-64 y y  65-80 y y 81-96 y y  97-112 y y 113-128 y y 129-144 y y 145-160 y y 161-176 y y177-192 y y 193-208 y y 209-224 y y 225-240 y y 241-256 y y Wiper BSection tapping points Step No. 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 1 1-16 y y  17-32 y y  33-48 y y  49-64 y y  65-80 y y  81-96 y y  97-112y y 113-128 y y 129-144 y y 145-160 y y 161-176 y y 177-192 y y 193-208y y 209-224 y y 225-240 y y 241-256 y “y” means the connection to A-endor to B-end is selected (switch is on)

The first column with heading “Wiper Step No.” indicates the groups (inrows) of steps (by their numbers) that can be accessed by the Wiperterminal. The corresponding A-end and B-end terminals are given for eachrow in columns 2 and 3, respectively. For instance, the group of steps49 to 64 may be accessed, when fixed contacts A0, B15, and the movingcontact pair A3-B3 are selected. The specific step selection depends onthe “wiper” contact setting. Table 3 shows how the tapping points in Wsection should be selected as a function of Local Wiper Step Number. TheLocal Wiper Step Number is the resistance in unit of R between terminalW and the end of W-section in the direction of B-end.

TABLE 3 The Selection of W tapping points vs. Local Wiper Step NumbersLocal W Section tapping points WS No. 0 1 2 3 4 5 6 7 8 9 10 11 12 13 1415 16 Accessible Wiper Step No. 1 y 1, 17, 33, 49, 65, 81, 97, 113, 129,145, 161, 177, 193, 209, 225, 241 2 y y 2, 18, 34, 50, 66, 82, 98, 114,130, 146, 162, 178, 194, 210, 226, 242 3 y 3, 19, 35, 51, 67, 83, 99,115, 131, 147, 163, 179, 195, 211, 227, 243 4 y 4, 20, 36, 52, 68, 84,100, 116, 132, 148, 164, 180, 196, 212, 228, 244 5 y 5, 21, 37, 53, 69,85, 101, 117, 133, 149, 165, 181, 197, 213, 229, 245 6 y 6, 22, 38, 54,70, 86, 102, 118, 134, 150, 166, 182, 198, 214, 230, 246 7 y 7, 23, 39,55, 71, 87, 103, 119, 135, 151, 167, 183, 199, 215, 231, 247 8 y 8, 24,40, 56, 72, 88, 104, 120, 136, 152, 168, 184, 200, 216, 232, 248 9 y 9,25, 41, 57, 73, 89, 105, 121, 137, 153, 169, 185, 201, 217, 233, 249 10y 10, 26, 42, 58, 74, 90, 106, 122, 138, 154, 170, 186, 202, 218, 234,250 11 y 11, 27, 43, 59, 75, 91, 107, 123, 139, 155, 171, 187, 203, 219,235, 251 12 y 12, 28, 44, 60, 76, 92, 108, 124, 140, 156, 172, 188, 204,220, 236, 252 13 y 13, 29, 45, 61, 77, 93, 109, 125, 141, 157, 173, 189,205, 221, 237, 253 14 y 14, 30, 46, 62, 78, 94, 110, 126, 142, 158, 174,190, 206, 222, 238, 254 15 y 15, 31, 47, 63, 79, 95, 111, 127, 143, 159,175, 191, 207, 223, 239, 255 16 y 16, 32, 48, 64, 80, 96, 112, 128, 144,160, 176, 192, 208, 224, 240, 256 “y” means the connection is selected(the switch closed)

The first column with heading “Local Wiper Step No.” indicates thegroups (in rows) that may be accessed by the Wiper tapping points. Thecorresponding wiper tapping points are given for each row in column 2.The accessible Wiper Step Numbers are given in column 3. For instance,group 3 may be accessed, when W13 is selected, and step 3, 19, 35, 51,67, 83, 99, 115, 131, 147, 163, 179, 195, 211, 227, or 243 may be set atthe wiper depending on the A-B pair selection. In general, the WiperStep Numbers is the sum of 16×(16−A-B pair tapping position) and WiperLocal Step number.

Within the specific scheme described above, the values of r₀ to r₁₅ maybe set accordingly as follows:

TABLE 4 Resistor Value R Resistance of unit resistor (R − r₀) Resistanceof end-resistors of W-section r₀ Fraction of R r₁ r₀*[(1 − r₀/(16R)) r₂r₀*[(1 − r₀/(32R)) r₃ r₀*[(1 − r₀/(48R)) r₄ r₀*[(1 − r₀/(64R)) r₅ r₀*[(1− r₀/(80R)) r₆ r₀*[(1 − r₀/(96R)) r₇ r₀*[(1 − r₀/(112R)) r₈ r₀*[(1 −r₀/(128R)) r₉ r₀*[(1 − r₀/(144R)) r₁₀ r₀*[(1 − r₀/(160R)) r₁₁ r₀*[(1 −r₀/(176R)) r₁₂ r₀*[(1 − r₀/(192R)) r₁₃ r₀*[(1 − r₀/(208R)) r₁₄ r₀*[(1 −r₀/(224R)) r₁₅ r₀*[(1 − r₀/(240R))

These values are chosen, so that the resistances from A-end to the wipertapping position and from wiper tapping position to B-end will bemultiples of R.

3. Step Change

To change from one step to the next, the connecting point may be movedup or step down the tapping points one step a time in the wiperconnection section. To avoid voltage spike in the output, the switch tothe target point is first closed, then the switch to the starting pointis open.

4. Range Change

When the limits of connecting points, W_0 and W_16, are reached, then anew A-B pair needs to be switched in and the connecting point in the Wsections to be moved to the opposite limits. This Range Change requiresmoving A-end and B-end connection from one A-B pair tapping points tothe next. Smooth range change may be achieved, if internal nodes, calledeffective tapping points, between the original tapping point and thetarget tapping point can be reached effectively by adjusting theresistance of the associated switches.

This is depicted in FIG. 5 for a system with 16 fine steps (W-section).Seven effective tapping points have been added between the tappingpoints (solid arrows) of A(i) and A(i+1), and B(i) and B(i+1), torepresent where the A-end and B-end terminals may effectively tap intothe resistor string by controlling the resistance of the associatedswitches appropriately. Depending on the design of the switches (such ascontrol, gate drive, etc.), more or less effective tapping points may becreated, or may even be made continuous.

Using the system above, and assuming substantially all of the effectivetapping points are assessable, the operation of range change may beconducted as follows:

4.1 Change from Pair A(i)-B(i) to Pair A(i+1)-B(i+1)

This is so called the “range move down” (or moving towards B-end) case.A sequence of operations is defined as follows. 1) Initial condition:A-end terminal is connected to A(i). Wiper terminal is connected to W_0.B-end terminal is connected to B(i). 2) Connect B-terminal to B(i+1).{Switch fully on}, while keep the connection to B(i). 3) Turn SwitchA(i+1) partially on, such that A-end is effectively connected to Eff.{T.P. Ra=14R}. 4) Connect W-terminal to W_1. Then release the connectionto W_0. 5) Turn Switch B(i) partialy off, such that B-end is effectivelyconnected to Eff. {T.P. Rb=2R}. 6) Connect W-terminal to W_2. Thenrelease the connection to W_1. 7) Turn Switch A(i+1) on more, such thatA-end is effectively connected to Eff. {T.P. Ra=12R}. 8) ConnectW-terminal to W_2. Then release the connection to W_1. 9) Turn SwitchB(i) off, such that B-end is effectively connected to Eff. {T.P. Rb=4R}.10) Connect W-terminal to W_3. Then release the connection to W_2. 11)In this manner, the steps similar to 3) through 6) are repeatedlyapplied, such that A-end terminal is effectively connected to Eff. (T.P.Ra=2R) Wiper terminal is connected to W_14. B-end terminal iseffectively connected to Eff. (T.P. Rb=14R} Finally, the A(i+1)-B(i+1)may be reached. 12) Turn Switch A(i+1) fully on. Then release theconnection to A(i). 13) Connect W-terminal to W_15. Then release theconnection to W_14. 14) Turn Switch B(i) fully off, such that B-end isonly connected to B(i+1). 15) Connect W-terminal to W_16. Then releasethe connection to W_15.

The simulation (not include the capacitance effect) of “range move down”is shown in FIG. 6.

4.2 Change from Pair the A(i+1)-B(i+1) to Pair A(i)-B(i)

This is so called the “range move up” (or moving towards A-end) case.This may be done in a similar fashion as in section 4.1, but reversingthe role of A (switches, effective T. P . . . ) and B.

4.3 Switch Resistance and Conductance

The resistance value of the switch may be calculated for the targeteffective tapping points. Thus, for the example case,

1. The A(i+1) Switch

TABLE 5A The variable Switch Resistance {A(i + 1)} Resultant SwitchResistance Switch Conductance Effective T.P. Resistance [a] [a]{Ra(i + 1) = 14R} 14 112.0 0.0089 {Ra(i + 1) = 12R} 12 48.0 0.0208{Ra(i + 1) = 10R} 10 26.7 0.0375 {Ra(i + 1) = 8R} 8 16.0 0.0625{Ra(i + 1) = 6R} 6 9.6 0.1042 {Ra(i + 1) = 4R} 4 5.3 0.1875 {Ra(i + 1) =2R} 2 2.3 0.4375

2. The B(i) Switch

TABLE 5B The variable Switch Resistance {B(i)} Resultant SwitchEffective T.P. Resistance Resistance Conductance {Ra(i + 1) = 2R} 2 2.30.4375 {Ra(i + 1) = 4R} 4 5.3 0.1875 {Ra(i + 1) = 6R} 6 9.6 0.1042{Ra(i + 1) = 8R} 8 16.0 0.0625 {Ra(i + 1) = 10R} 10 26.7 0.0375{Ra(i + 1) = 12R} 12 48.0 0.0208 {Ra(i + 1) = 14R} 14 112.0 0.0089

5. General Solution

In this architecture, a potentiometer is organized in ranges of equaldistance, and fine steps of equal space. Explicitly, if the total numberof programmable steps of the potentiometer is N, then N equals L timesM, where L is the number of range unit and M is the number of finesteps. Moreover, there should be (L−1) units of resistors in A section,(L−1) units in B section and M units in W section with unit resistanceequal to LR, LR, and R, respectively. Accordingly, the total number ofswitches is 2×(L−1)+M+1. To minimize the number of switches, L_minshould be equal to $\sqrt{\frac{N}{2}}.$

Since only the integer is allowed as solution and N/L-min also has to bean integer, the solutions in L for minimal number of switches will besomething close to (N/2), and may come in pairs. An example of a256-step potentiometer solution is shown below.

TABLE 6 The Solution for Minimum Number of Switches in the case of 256steps Number of resistor units Number of Range units in W Number ofswitches L M 2 × (L − 1) = M = 1 4 64 71 8 32 47 16 16 47 32 8 71

What is claimed is:
 1. An impedance network, comprising: a plurality ofimpedance elements; at least one end terminal; a first plurality ofswitching elements selectively providing tap positions to the at leastone end terminal, selectable at a first specified increment of impedanceelements in the plurality of impedance elements; a wiper terminal; asecond plurality of switching elements selectively providing a tappositions to the wiper terminal, selectable at a second specifiedincrement of impedance elements in the plurality of impedance elements;and a third plurality of switching elements selectively providing tappositions to the wiper terminal, selectable at a second specifiedincrement of impedance elements in the plurality of impedance elementsto substantially reduce the direct connection of the first plurality ofswitching elements to the at least one terminal.
 2. An impedancenetwork, comprising: a plurality of impedance elements; at least one endterminal; a first plurality of switching elements selectively providingtap positions to the at least one end terminal, selectable at a firstspecified increment of impedance elements in the plurality of impedanceelements; a wiper terminal; a second plurality of switching elementsselectively providing a tap positions to the wiper terminal, selectableat a second specified increment of impedance elements in the pluralityof impedance elements; and a third plurality of switching elementsselectively providing tap positions to the wiper terminal, selectable ata third specified increment of impedance elements in the plurality ofimpedance elements to substantially reduce the direct connection of thefirst plurality of switching elements to the at least one terminal. 3.The network of claim 2, wherein the third specified increment is smallerthan the second increment.
 4. A method for configuring an impedancenetwork, comprising: providing a plurality of impedance elements;providing at least one end terminal and a wiper terminal; firstselectively providing tap positions to the at least one end terminal,selectable at a first specified increment of impedance elements in thenetwork; second selectively providing a tap positions to the wiperterminal, selectable at a second specified increment of impedanceelements in the network; and third selectively providing tap positionsto the wiper terminal, selectable at a third specified increment toprotect the at least one end terminal.